Multipole magnets and particularly quadrupole magnets have been found useful for a variety of applications including, for example, focusing charged particle beams. Conventionally, electromagnets have been used for such multipole configurations because of the limitations of the field strength of permanent multipole magnets and because the field strength of electric magnets could be easily varied by controlling the coil current whereas the field strength of permanent magnets is fixed.
Rare earth-cobalt (REC) materials have renewed interest in permanent magnet multipoles. Most of the work has been done with respect to quadrupole magnets. For the past several years there has been considerable effort in developing permanent magnet quadrupoles for replacing electromagnets, particularly in applications such as the drift tubes in proton linacs.
Recently a new design for permanent magnet quadrupoles was described. See, for instance, Halbach, "Strong Rare Earth Cobalt Quadrupoles", IEEE Trans, Nucl. Sci., (June 1979), Holsinger et al., "A New Generation of Samarium-Cobalt Quadrupole Magnets for Particle Beam Focusing Applications", Proc. Fourth Int. Workshop REC Perm. Mag. and Appl., (1979) and Halbach, "Design of Permanent Multipole Magnets With Oriented Rare Earth Cobalt Material", Nucl. Inst. Meth., 169, pp. 1-10 (1980), which are hereby incorporated by reference. The new design for REC quadrupoles allows construction of compact quadrupoles with magnet aperture fields of at least 1.2 tesla (T) with presently available materials. The development of high field permanent magnet quadrupoles opens up their use in a variety of beam line applications. However, to realize the advantages of permanent magnet quadrupoles in large aperture beam line magnets, two significant problems need to be solved: (1) the quadrupole focusing strength must be adjustable in most applications, and (2) the cost of the REC pieces must be controlled so that the total cost of the quadrupole assembly will be comparable to that of an electromagnet including the power supply.
Various approaches have been suggested to adjust the quadrupole strength of these permanent magnet quadrupoles by rotation of the quadrupoles, but these typically have the undesirable feature of coupling the motion in the two transverse directions. Thus, it remains desirable to obtain variable strength permanent magnet quadrupoles for beam line applications wherein the quadrupoles produce no coupling of the beam line motion in the two transverse directions.